Clones of cultured mouse malignant plasma cells (myelomas) which are variant in their ability to synthesize, assemble, and secrete Ig will be isolated. Variants will be sought which: 1) have lost the ability to synthesize one of the three polypeptide chains; 2) produce chains with altered primary structure; 3) are altered in binding of antigen; 4) assemble Ig at different rate or via an altered pathway; 5) process their Ig in an altered fashion; or 6) have altered surface Ig. These variants will be characterized with respect to the frequency with which they occur and the structural and cellular changes involved. Isolation and characterization of these variants should give information about control and regulation in immunoglobulin producing cells, the structure-function relationships in the immunoglobulin molecule, the genetic organization of the immunoglobulin locus, and the cellular functions in immunoglobulin biosynthesis. Detailed molecular characterization should define the precise mechanisms of variation. Antigen binding variants should help clarify the nature of antigen binding by Ig and should give some insight into the feasibility of the somatic generation of antibody diversity.